Continuous spectrum.
I think we may also claim the continuous spectrum in the Aurora in further proof of water-vapour (see Plate XV. spectrum 7). The continuous spectrum of the Aurora is also, to my observation, more local and dense in the spectroscope than the glow generally seen between the lines or bands in gas-spectra.
Violet-pole spectrum discussed. Most spectra have a general as well as special character.
B. The question of the violet-pole spectrum. Here I make the remark that in comparing other spectra with that of the Aurora, it is, I think, too much the practice to trust to the coincidence (more or less perfect) of one or perhaps two lines out of many; whereas we know by experience that most spectra have so well-marked a general as well as special character that, when once seen, they are recognized afterwards with the greatest ease and without measurements. An experience and proof of this is found in a set of “Photographed Spectra” which the Autotype Company have reproduced for me.
Coincidence of one or two lines not sufficient to establish identity.
Of course no two given spectra can be considered identical unless their principal lines coincide; but, on the other hand, the coincidence of one or two lines out of many, without other features, cannot be satisfactorily or conclusively held to establish identity.
Ångström’s compared spectra.
In Professor Herschel’s letter (Phil. Mag. ser. 4, vol. xlix. p. 71), Professor Ångström’s representation of the “spectrum of the glow discharge round the negative pole of air-vacuum tubes” is given, in comparison with the Aurora-lines and those of olefiant gas. This illustration is here introduced.
Ångström’s representation of the Spectrum of the glow discharge round the negative pole of Air-vacuum tubes, and its comparison with the Spectrum of the Aurora.